1. Field of the Invention
The present invention relates to a low-battery state detecting system and method for suitable use in a personal computer using a battery as a power source.
2. Description of the Related Art
There is known a personal computer using a battery as a power source and including a power controller capable of monitoring the residual capacity of the battery. The power controller normally includes a 1-chip microcomputer and performs data communication with a host CPU through a serial line, thereby supplying necessary power.
In the prior art, the state ("low-battery state"), wherein the residual capacity of the battery so decreases that it may be difficult to normally operate the personal computer, is detected when the current output from the battery and the battery voltage are monitored and found to fail in a predetermined condition.
The method of detecting the low-battery state will now be described in detail with reference to FIG. 1 or a graph showing battery characteristics. FIG. 1 illustrates how the battery voltage varies with the lapse of time. The ordinate indicates battery voltage, and the abscissa indicates time. In FIG. 1, three curves I, II and III represent the variations of battery voltages when electric currents of 200 mA, 400 mA and 600 mA are output from the battery. As is obvious from FIG. 1, the greater the output current from the battery, the sooner the battery voltage lowers.
Point A on the ordinate represents the minimum voltage for normal operation of the personal computer. Point C on the abscissa represents the time when the battery voltage lowers to point A in the case where the current is 600 mA. Point B on the abscissa represents a reference point for determining whether the battery is in the low-battery state. When the maximum value of the electric current output from the battery is 600 mA, the normal operation of the personal computer is ensured at least in the time period of points C-B after the low-battery state is determined.
In the prior art, the current output from the battery and the battery voltage are monitored, and when the battery voltage reaches point B (B'-I, B'-II, B'-III), the low-battery state is determined. In the battery-powered personal computer, the longer the time of the battery-powered operation, the more desirable the computer. For this purpose, it is an essential factor to exactly detect the low-battery state. This enables the battery to be used up to a maximum capacity, thereby lengthening the time of the battery-powered operation. In a conventional technique, the low-battery state cannot be detected precisely, because of battery variations. This problem will now be explained with reference to FIG. 2 of a graph of battery characteristics.
FIG. 2 shows variations in battery voltages when the battery is charged in different charge time periods. Point A on the ordinate represents the minimum voltage for normal operation of the personal computer. Point B' on the abscissa represents a reference point for determining whether or not the battery is in the low-battery state. The time period (Cn-Bn, n=1, 2, 3, 4) between the determination of the low-battery state and the failure of the normal operation of the personal computer, increases with as the charge time of the battery. Thus, in the conventional technique, the battery remains sufficient for normal operation of the personal computer, in some cases, despite the fact that the low-battery state has been determined. However, the battery cannot be used efficiently.